Sudden cardiac death (SCD) is a major public health problem that accounts for more than half of all cardiovascular deaths. SCD takes the lives of approximately 450,000 people in the United States each year, more than lung cancer, breast cancer, stroke, and AIDS combined. Most cases of SCD are due to ventricular arrhythmias and there is often an element of underlying ischemic heart disease. Ventricular tachycardia (VT) and ventricular fibrillation (VF) are different types of ventricular arrhythmias. VT is an abnormally fast ventricular heart rhythm which is, by itself, typically not fatal. VF is a chaotic ventricular heart rhythm which produces little or no net blood flow from the heart, such that there is little or not net blood flow to the brain and other organs. VF, if not terminated, results in death. Patient groups most at risk of ventricular arrhythmias leading to SCD include those with an acute or chronic myocardial infarction. Accordingly, deaths from SCDs may be lowered by preventing the specific heart rhythm disturbances (ventricular arrhythmias) associated with it.
Different treatment options exist for SCD. The most common treatment includes implantable cardiac defibrillators (ICD) and drug therapy. ICDs have been available in the United States since the mid-1980s and have a well-documented success rate in decreasing the rate of death of patients at high risk for SCD. A major trial conducted by the U.S. National Institutes of Health (the Anti-arrhythmics Versus Implantable Defibrillator or AVID trial) compared therapy with the best available anti-arrhythmic drugs with ICD therapy for patients with spontaneous ventricular tachycardia or ventricular fibrillation. The overall death rate in the ICD patient group was found to be 39% lower than the death rate of patients treated with anti-arrhythmic drugs after only 18 months mean follow-up.
An ICD has two basic components: the ICD generator and the lead system for pacing and shock delivery to which it is connected. An ICD generator contains sensing circuits, memory storage, capacitors, voltage enhancers, a telemetry module, and a control microprocessor. Advances in miniaturization and complexity in all of these components have permitted a tremendous reduction in size of the generator itself despite increased functionality, such as added programming options, anti-tachycardia pacing, single- and dual-chamber rate-responsive pacing for bradycardia, biphasic defibrillation waveforms, enhanced arrhythmia detection features, and innovations in lead systems.
Current ICD technology, however, provides for the detection and recognition of an arrhythmia based on the sensed heart rate once it has already started. This leaves very little time to protect the individual from death resulting from SCD. Although there have been several attempts at developing new technology for predicting the onset of a cardiac arrhythmia, many of these methods and systems appear to rely primarily on events occurring within the heart, such as sensed heart rate and electrocardiography (ECG). For example, U.S. Pat. No. 6,308,094 discloses a method and device for predicting cardiac arrhythmias by gathering and processing electrocardiographic data, such as intervals between heart beats (RR-series) or other heart signals, to predict the occurrence of a cardiac arrhythmia. U.S. Pat. No. 6,516,219 discloses a method and apparatus for forecasting arrhythmia based on real-time intact intracardiac electrograms.